17
17

Sep 20, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 17

######
favorite 0

######
comment 0

In this work, a generalization of the Mazur-Mottola gravastar model is explored, by considering a matching of an interior solution governed by the dark energy equation of state, $\omega\equiv p/ \rho

Source: http://arxiv.org/abs/gr-qc/0612030v1

27
27

Sep 20, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 27

######
favorite 0

######
comment 0

Firstly, we review the pointwise and averaged energy conditions, the quantum inequality and the notion of the ``volume integral quantifier'', which provides a measure of the ``total amount'' of energy condition violating matter. Secondly, we present a specific metric of a spherically symmetric traversable wormhole in the presence of a generic cosmological constant, verifying that the null and the averaged null energy conditions are violated, as was to be expected. Thirdly, a pressureless dust...

Source: http://arxiv.org/abs/gr-qc/0410087v1

40
40

Sep 23, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 40

######
favorite 0

######
comment 0

We review recent work on wormhole geometries in the context of modified theories of gravity, in particular, in f(R) gravity and with a nonminimal curvature-matter coupling, and in the recently proposed hybrid metric-Palatini theory. In principle, the normal matter threading the throat can be shown to satisfy the energy conditions and it is the higher order curvatures terms that sustain these wormhole geometries. We also briefly review the conversion of wormholes into time-machines, explore...

Source: http://arxiv.org/abs/1212.1006v1

19
19

Sep 20, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 19

######
favorite 0

######
comment 0

It has recently been shown that traversable wormholes may be supported by phantom energy. In this work phantom wormhole geometries are modelled by matching an interior traversable wormhole solution, governed by the equation of state $p=\omega \rho$ with $\omega

Source: http://arxiv.org/abs/gr-qc/0506001v2

16
16

Sep 22, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 16

######
favorite 0

######
comment 0

We match an interior solution of a spherically symmetric traversable wormhole to a unique exterior vacuum solution, with a generic cosmological constant, at a junction interface, and the surface stresses on the thin shell are deduced. In the spirit of minimizing the usage of exotic matter we determine regions in which the weak and null energy conditions are satisfied on the junction surface. The characteristics and several physical properties of the surface stresses are explored, namely,...

Source: http://arxiv.org/abs/gr-qc/0409018v2

21
21

Sep 21, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 21

######
favorite 0

######
comment 0

A central theme in cosmology is the perplexing fact that the Universe is undergoing an accelerating expansion. The latter, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational field equations. Several candidates, responsible for this expansion, have been proposed in the literature, in particular, dark energy models and modified gravity, amongst others. In this paper, we explore the possibility that the late-time...

Source: http://arxiv.org/abs/1011.6176v1

27
27

Sep 19, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 27

######
favorite 0

######
comment 0

The brane cosmology scenario is based on the idea that our Universe is a 3-brane embedded in a five-dimensional bulk. In this work, a general class of braneworld wormholes is explored with $R\neq 0$, where $R$ is the four dimensional Ricci scalar, and specific solutions are further analyzed. A fundamental ingredient of traversable wormholes is the violation of the null energy condition (NEC). However, it is the effective total stress energy tensor that violates the latter, and in this work, the...

Source: http://arxiv.org/abs/gr-qc/0701133v2

19
19

Sep 18, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 19

######
favorite 0

######
comment 0

A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition. In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these non-standard wormhole geometries, fundamentally different from their...

Source: http://arxiv.org/abs/1211.1170v1

22
22

Sep 24, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 22

######
favorite 0

######
comment 0

A fundamental ingredient in wormhole physics is the presence of exotic matter, which involves the violation of the null energy condition. Although a plethora of wormhole solutions have been explored in the literature, it is useful to find geometries that minimize the usage of exotic matter. In the context of modified gravity, it has also been shown that the normal matter can be imposed to satisfy the null energy condition, and it is the higher order curvature terms, interpreted as a...

Source: http://arxiv.org/abs/1112.6333v1

18
18

Sep 22, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 18

######
favorite 0

######
comment 0

A possible candidate for the present accelerated expansion of the Universe is ''phantom energy'', which possesses an equation of state of the form $\omega\equiv p/\rho

Source: http://arxiv.org/abs/gr-qc/0603091v1

16
16

Sep 23, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 16

######
favorite 0

######
comment 0

In this work, a general class of wormhole geometries in conformal Weyl gravity is analyzed. A wide variety of exact solutions of asymptotically flat spacetimes is found, in which the stress energy tensor profile differs radically from its general relativistic counterpart. In particular, a class of geometries is constructed that satisfies the energy conditions in the throat neighborhood, which is in clear contrast to the general relativistic solutions.

Source: http://arxiv.org/abs/0801.4401v3

17
17

Sep 22, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 17

######
favorite 0

######
comment 0

The authors of the paper ''On Anisotropic Dark Energy Stars'', arXiv:0803.2508 [gr-qc], consider the equations of state $p_r=\omega \rho$ and $p_t=\omega \rho$, ignoring the fact that this implies an isotropic pressure, which places strict restrictions on the values of $\omega$. The authors then argue for an anisotropic pressure fluid throughout their work, and apply these equations of state to the energy conditions, consequently obtaining incorrect intervals for the parameter $\omega$. This...

Source: http://arxiv.org/abs/0805.2309v2

37
37

Jul 24, 2013
07/13

by
Francisco S. N. Lobo

texts

######
eye 37

######
favorite 0

######
comment 0

Modern astrophysical and cosmological models are faced with two severe theoretical difficulties, that can be summarized as the dark energy and the dark matter problems. Relative to the former, it has been stated that cosmology has entered a 'golden age', in which high-precision observational data have confirmed with startling evidence that the Universe is undergoing a phase of accelerated expansion. Several candidates, responsible for this expansion, have been proposed in the literature, in...

Source: http://arxiv.org/abs/0807.1640v1

23
23

Sep 19, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 23

######
favorite 0

######
comment 0

The van der Waals quintessence equation of state is an interesting scenario for describing the late universe, and seems to provide a solution to the puzzle of dark energy, without the presence of exotic fluids or modifications of the Friedmann equations. In this work, the construction of inhomogeneous compact spheres supported by a van der Waals equation of state is explored. These relativistic stellar configurations shall be denoted as {\it van der Waals quintessence stars}. Despite of the...

Source: http://arxiv.org/abs/gr-qc/0610118v2

20
20

Sep 21, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 20

######
favorite 0

######
comment 0

The gravastar picture is an alternative model to the concept of a black hole, where there is an effective phase transition at or near where the event horizon is expected to form, and the interior is replaced by a de Sitter condensate. In this work, a generalization of the gravastar picture is explored, by considering a matching of an interior solution governed by the dark energy equation of state, $\omega\equiv p/ \rho

Source: http://arxiv.org/abs/gr-qc/0508115v3

14
14

Sep 23, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 14

######
favorite 0

######
comment 0

The General Theory of Relativity has been an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes, gravitational radiation to the cosmological models, predicting a primordial beginning, namely the big-bang. All these solutions have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry...

Source: http://arxiv.org/abs/0710.4474v1

29
29

Sep 23, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 29

######
favorite 0

######
comment 0

The conceptual definition and understanding of the nature of time, both qualitatively and quantitatively is of the utmost difficulty and importance, and plays a fundamental role in physics. Physical systems seem to evolve in paths of increasing entropy and of complexity, and thus, the arrow of time shall be explored in the context of thermodynamic irreversibility and quantum physics. In Newtonian physics, time flows at a constant rate, the same for all observers; however, it necessarily flows...

Source: http://arxiv.org/abs/0710.0428v2

37
37

Sep 18, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 37

######
favorite 0

######
comment 0

It has been suggested that a possible candidate for the present accelerated expansion of the Universe is ''phantom energy''. The latter possesses an equation of state of the form $\omega\equiv p/\rho

Source: http://arxiv.org/abs/gr-qc/0502099v2

18
18

Sep 23, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 18

######
favorite 0

######
comment 0

The generalized Chaplygin gas (GCG) is a candidate for the unification of dark energy and dark matter, and is parametrized by an exotic equation of state given by $p_{ch}=-A/\rho_{ch}^{\alpha}$, where $A$ is a positive constant and $0

Source: http://arxiv.org/abs/gr-qc/0511003v2

19
19

Sep 18, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 19

######
favorite 0

######
comment 0

Applying the Darmois-Israel thin shell formalism, we construct static and dynamic thin shells around traversable wormholes. Firstly, by applying the cut-and-paste technique we apply a linearized stability analysis to thin-shell wormholes in the presence of a generic cosmological constant. We find that for large positive values of the cosmological constant, i.e., the Schwarzschild-de Sitter solution, the regions of stability significantly increase relatively to the Schwarzschild case, analyzed...

Source: http://arxiv.org/abs/gr-qc/0401083v1

46
46

Sep 21, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 46

######
favorite 0

######
comment 0

The conceptual definition and understanding of time, both quantitatively and qualitatively is of the utmost difficulty and importance. As time is incorporated into the proper structure of the fabric of spacetime, it is interesting to note that General Relativity is contaminated with non-trivial geometries which generate closed timelike curves. A closed timelike curve (CTC) allows time travel, in the sense that an observer that travels on a trajectory in spacetime along this curve, may return to...

Source: http://arxiv.org/abs/1008.1127v1

47
47

Sep 18, 2013
09/13

by
Francisco S. N. Lobo

texts

######
eye 47

######
favorite 0

######
comment 0

We explore the possibility that traversable wormholes be supported by specific equations of state responsible for the present accelerated expansion of the Universe, namely, phantom energy, the generalized Chaplygin gas, and the van der Waals quintessence equation of state.

Source: http://arxiv.org/abs/gr-qc/0611150v1

19
19

Sep 22, 2013
09/13

by
Francisco S. N. Lobo; Remo Garattini

texts

######
eye 19

######
favorite 0

######
comment 0

In this work, we find exact gravastar solutions in the context of noncommutative geometry, and explore their physical properties and characteristics. The energy density of these geometries is a smeared and particle-like gravitational source, where the mass is diffused throughout a region of linear dimension $\sqrt{(\alpha)}$ due to the intrinsic uncertainty encoded in the coordinate commutator. These solutions are then matched to an exterior Schwarzschild spacetime. We further explore the...

Source: http://arxiv.org/abs/1004.2520v1

27
27

Jul 20, 2013
07/13

by
Nadiezhda Montelongo Garcia; Francisco S. N. Lobo

texts

######
eye 27

######
favorite 0

######
comment 0

Wormhole geometries in curvature-matter coupled modified gravity are explored, by considering an explicit nonminimal coupling between an arbitrary function of the scalar curvature, R, and the Lagrangian density of matter. It is the effective stress-energy tensor containing the coupling between matter and the higher order curvature derivatives that is responsible for the null energy condition violation, and consequently for supporting the respective wormhole geometries. The general restrictions...

Source: http://arxiv.org/abs/1007.3040v2

24
24

Sep 19, 2013
09/13

by
Francisco S. N. Lobo; Miguel A. Oliveira

texts

######
eye 24

######
favorite 0

######
comment 0

In this work, we construct traversable wormhole geometries in the context of f(R) modified theories of gravity. We impose that the matter threading the wormhole satisfies the energy conditions, so that it is the effective stress-energy tensor containing higher order curvature derivatives that is responsible for the null energy condition violation. Thus, the higher order curvature terms, interpreted as a gravitational fluid, sustain these non-standard wormhole geometries, fundamentally different...

Source: http://arxiv.org/abs/0909.5539v2

30
30

Sep 19, 2013
09/13

by
Christian G. Boehmer; Francisco S. N. Lobo

texts

######
eye 30

######
favorite 0

######
comment 0

The Florides solution, proposed as an alternative to the interior Schwarzschild solution, represents a static and spherically symmetric geometry with vanishing radial stresses. It is regular at the center, and is matched to an exterior Schwarzschild solution. The specific case of a constant energy density has been interpreted as the field inside an Einstein cluster. In this work, we are interested in analyzing the geometry throughout the permitted range of the radial coordinate without matching...

Source: http://arxiv.org/abs/gr-qc/0703024v1

24
24

Sep 21, 2013
09/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 24

######
favorite 0

######
comment 0

We generalize the $f(R)$ type gravity models by assuming that the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar $R$ and of the matter Lagrangian $L_m$. We obtain the gravitational field equations in the metric formalism, as well as the equations of motion for test particles, which follow from the covariant divergence of the energy-momentum tensor. The equations of motion for test particles can also be derived from a variational principle in the particular case...

Source: http://arxiv.org/abs/1008.4193v2

34
34

Sep 22, 2013
09/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 34

######
favorite 0

######
comment 0

We investigate the interaction between dark energy and dark matter in the framework of irreversible thermodynamics of open systems with matter creation/annihilation. We consider dark energy and dark matter as an interacting two component (scalar field and "ordinary" dark matter) cosmological fluid in a homogeneous spatially flat and isotropic Friedmann-Robertson-Walker (FRW) Universe. The thermodynamics of open systems as applied together with the gravitational field equations to the...

Source: http://arxiv.org/abs/1210.3617v2

28
28

Sep 23, 2013
09/13

by
Remo Garattini; Francisco S. N. Lobo

texts

######
eye 28

######
favorite 0

######
comment 0

In this work, we consider the possibility that wormhole geometries are sustained by their own quantum fluctuations, in the context of modified dispersion relations. More specifically, the energy density of the graviton one-loop contribution to a classical energy in a wormhole background is considered as a self-consistent source for wormholes. In this semi-classical context, we consider specific choices for the Rainbow's functions and find solutions for wormhole geometries in the cis-planckian...

Source: http://arxiv.org/abs/1111.5729v1

18
18

Sep 21, 2013
09/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 18

######
favorite 0

######
comment 0

A two-fluid dark matter model, in which dark matter is represented as a two-component fluid thermodynamic system, without interaction between the constituent particles of different species, and with each distinct component having a different four-velocity, was recently proposed in Harko & Lobo, [Phys. Rev. D83, 124051 (2011)]. In the present paper we further investigate the two-fluid dark matter model, by assuming that the two dark matter components are pressureless, non-comoving fluids....

Source: http://arxiv.org/abs/1104.2674v2

19
19

Sep 18, 2013
09/13

by
Francisco S. N. Lobo; Tiberiu Harko

texts

######
eye 19

######
favorite 0

######
comment 0

We consider a maximal extension of the Hilbert-Einstein action and analyze several interesting features of the theory. More specifically, the motion is non-geodesic and takes place in the presence of an extra force. These models could lead to some major differences, as compared to the predictions of General Relativity or other modified theories of gravity, in several problems of current interest, such as cosmology, gravitational collapse or the generation of gravitational waves. Thus, the study...

Source: http://arxiv.org/abs/1211.0426v1

23
23

Sep 22, 2013
09/13

by
Remo Garattini; Francisco S. N. Lobo

texts

######
eye 23

######
favorite 0

######
comment 0

In this work, we find exact wormhole solutions in the context of noncommutative geometry, and further explore their physical properties and characteristics. The energy density of these wormhole geometries is a smeared and particle-like gravitational source, where the mass is diffused throughout a region of linear dimension $\sqrt{\alpha}$ due to the intrinsic uncertainty encoded in the coordinate commutator. Furthermore, we also analyze these wormhole geometries considering that the equation...

Source: http://arxiv.org/abs/0811.0919v2

17
17

Sep 19, 2013
09/13

by
Remo Garattini; Francisco S. N. Lobo

texts

######
eye 17

######
favorite 0

######
comment 0

A possible candidate for the late time accelerated expanding Universe is phantom energy, which possesses rather bizarre properties, such as the prediction of a Big Rip singularity and the violation of the null energy condition. The latter is a fundamental ingredient of traversable wormholes, and it has been shown that phantom energy may indeed sustain these exotic geometries. Inspired by the evolving dark energy parameter crossing the phantom divide, we consider in this work a varying equation...

Source: http://arxiv.org/abs/gr-qc/0701020v2

29
29

Sep 23, 2013
09/13

by
Remo Garattini; Francisco S. N. Lobo

texts

######
eye 29

######
favorite 0

######
comment 0

In this work, we explore the possibility that quantum fluctuations induce a topology change, in the context of Gravity's Rainbow. A semi-classical approach is adopted, where the graviton one-loop contribution to a classical energy in a background spacetime is computed through a variational approach with Gaussian trial wave functionals. The energy density of the graviton one-loop contribution, or equivalently the background spacetime, is then let to evolve, and consequently the classical energy...

Source: http://arxiv.org/abs/1303.5566v1

36
36

Jul 24, 2013
07/13

by
Lukas Hollenstein; Francisco S. N. Lobo

texts

######
eye 36

######
favorite 0

######
comment 0

In this work, exact solutions of static and spherically symmetric space-times are analyzed in f(R) modified theories of gravity coupled to nonlinear electrodynamics. Firstly, we restrict the metric fields to one degree of freedom, considering the specific case of g_tt\g_rr = -1. Using the dual P formalism of nonlinear electrodynamics an exact general solution is deduced in terms of the structural function H_P. In particular, specific exact solutions to the gravitational field equations are...

Source: http://arxiv.org/abs/0807.2325v2

23
23

Sep 21, 2013
09/13

by
Christian G. Boehmer; Francisco S. N. Lobo

texts

######
eye 23

######
favorite 0

######
comment 0

We analyze the stability of the Einstein static universe by considering homogeneous perturbations in the context of f(G) modified Gauss-Bonnet theories of gravity. By considering a generic form of f(G), the stability region of the Einstein static universe is parameterized by the linear equation of state parameter w=p/rho and the second derivative f"(G) of the Gauss-Bonnet term.

Source: http://arxiv.org/abs/0902.2982v3

14
14

Sep 23, 2013
09/13

by
Nadiezhda Montelongo Garcia; Francisco S. N. Lobo

texts

######
eye 14

######
favorite 0

######
comment 0

Recently, exact solutions of wormhole geometries supported by a nonminimal curvature-matter coupling were found, where the nonminimal coupling minimizes the violation of the null energy condition of normal matter at the throat. In this brief report, we present a solution where normal matter satisfies the energy conditions at the throat and it is the higher order curvature derivatives of the nonminimal coupling that is responsible for the null energy condition violation, and consequently for...

Source: http://arxiv.org/abs/1012.2443v1

49
49

Jul 20, 2013
07/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 49

######
favorite 0

######
comment 0

We consider a cosmological model in which the two major fluid components of the Universe, dark energy and dark matter, flow with distinct four-velocities. This cosmological configuration is equivalent to a single anisotropic fluid, expanding with a four-velocity that is an appropriate combination of the two fluid four-velocities. The energy density of the single cosmological fluid is larger than the sum of the energy densities of the two perfect fluids, i.e., dark energy and dark matter,...

Source: http://arxiv.org/abs/1304.0757v3

53
53

Jul 19, 2013
07/13

by
Francisco S. N. Lobo; Matt Visser

texts

######
eye 53

######
favorite 0

######
comment 0

"Warp drive" spacetimes are useful as "gedanken-experiments" that force us to confront the foundations of general relativity, and among other things, to precisely formulate the notion of "superluminal" communication. We verify the non-perturbative violation of the classical energy conditions of the Alcubierre and Natario warp drive spacetimes and apply linearized gravity to the weak-field warp drive, testing the energy conditions to first and second order of the...

Source: http://arxiv.org/abs/gr-qc/0406083v2

13
13

Sep 20, 2013
09/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 13

######
favorite 0

######
comment 0

The late-time cosmic acceleration may be due to infra-red modifications of General Relativity. In particular, we consider a maximal extension of the Hilbert-Einstein action and analyze several interesting features of the theory. Generally, the motion is non-geodesic and takes place in the presence of an extra force, which is orthogonal to the four-velocity. These models could lead to some major differences, as compared to the predictions of General Relativity or other modified theories of...

Source: http://arxiv.org/abs/1205.3284v1

29
29

Sep 21, 2013
09/13

by
Orfeu Bertolami; Francisco S. N. Lobo

texts

######
eye 29

######
favorite 0

######
comment 0

In this review paper, we consider the fundamental nature of time and causality, most particularly, in the context of the theories of special and general relativity. We also discuss the issue of closed timelike curves in the context of general relativity, and the associated paradoxes, the question of directionality of the time flow and, rather briefly, the problem of time in quantum gravity.

Source: http://arxiv.org/abs/0902.0559v1

36
36

Sep 24, 2013
09/13

by
Francisco S. N. Lobo; Matt Visser

texts

######
eye 36

######
favorite 0

######
comment 0

''Warp drive'' spacetimes are useful as ''gedanken-experiments'' and as a theoretician's probe of the foundations of general relativity. Applying linearized gravity to the weak-field warp drive, i.e., for non-relativistic warp-bubble velocities, we find that the occurrence of energy condition violations in this class of spacetimes is generic to the form of the geometry under consideration and is not simply a side-effect of the ''superluminal'' properties. Using the linearized construction it is...

Source: http://arxiv.org/abs/gr-qc/0412065v1

22
22

Sep 22, 2013
09/13

by
José P. Mimoso; Francisco S. N. Lobo

texts

######
eye 22

######
favorite 0

######
comment 0

Scalar-tensor (ST) gravity theories provide an appropriate theoretical framework for the variation of Newton's fundamental constant, conveyed by the dynamics of a scalar-field non-minimally coupled to the space-time geometry. The experimental scrutiny of scalar-tensor gravity theories has led to a detailed analysis of their post-newtonian features, and is encapsulated into the so-called parametrised post-newtonian formalism (PPN). Of course this approach can only be applied whenever there is a...

Source: http://arxiv.org/abs/1101.4405v1

18
18

Sep 22, 2013
09/13

by
José P. Mimoso; Francisco S. N. Lobo

texts

######
eye 18

######
favorite 0

######
comment 0

We investigate a static solution with an hyperbolic nature, characterised by a pseudo-spherical foliation of space. This space-time metric can be perceived as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs to the class of static vacuum solutions termed "a degenerate static solution of class A". In the present work we review its fundamental features, discuss the existence of generalised wormholes, and derive its extension to scalar-tensor gravity theories...

Source: http://arxiv.org/abs/1001.2643v1

23
23

Sep 22, 2013
09/13

by
Tiberiu Harko; Francisco S. N. Lobo

texts

######
eye 23

######
favorite 0

######
comment 0

The geodesic deviation equation, describing the relative accelerations of nearby particles, and the Raychaudhury equation, giving the evolution of the kinematical quantities associated with deformations (expansion, shear and rotation) are considered in the framework of modified theories of gravity with an arbitrary curvature-matter coupling, by taking into account the effects of the extra force. As a physical application of the geodesic deviation equation the modifications of the tidal forces...

Source: http://arxiv.org/abs/1210.8044v2

32
32

Sep 19, 2013
09/13

by
Christian G. Boehmer; Francisco S. N. Lobo

texts

######
eye 32

######
favorite 0

######
comment 0

Recently, Horava proposed a power counting renormalizable theory for (3+1)-dimensional quantum gravity, which reduces to Einstein gravity with a non-vanishing cosmological constant in IR, but possesses improved UV behaviors. In this work, we analyze the stability of the Einstein static universe by considering linear homogeneous perturbations in the context of an IR modification of Horava gravity, which implies a `soft' breaking of the `detailed balance' condition. The stability regions of the...

Source: http://arxiv.org/abs/0909.3986v2

23
23

Sep 21, 2013
09/13

by
Remo Garattini; Francisco S. N. Lobo

texts

######
eye 23

######
favorite 0

######
comment 0

We consider the possibility that wormhole geometries are sustained by their own quantum fluctuations, in the context of noncommutative geometry and Gravity's Rainbow models. More specifically, the energy density of the graviton one-loop contribution to a classical energy in a wormhole background is considered as a self-consistent source for wormholes. In this semi-classical context, we consider the effects of a smeared particle-like source in noncommutative geometry and of the Rainbow's...

Source: http://arxiv.org/abs/1301.0221v1

14
14

Sep 19, 2013
09/13

by
Francisco S. N. Lobo; Paulo Crawford

texts

######
eye 14

######
favorite 0

######
comment 0

We analyze the stability of generic spherically symmetric thin shells to linearized perturbations around static solutions. We include the momentum flux term in the conservation identity, deduced from the ''ADM'' constraint and the Lanczos equations. Following the Ishak-Lake analysis, we deduce a master equation which dictates the stable equilibrium configurations. Considering the transparency condition, we study the stability of thin shells around black holes, showing that our analysis is in...

Source: http://arxiv.org/abs/gr-qc/0507063v2

18
18

Sep 22, 2013
09/13

by
Francisco S. N. Lobo; Miguel A. Oliveira

texts

######
eye 18

######
favorite 0

######
comment 0

Recently, traversable wormhole geometries were constructed in the context of f(R) gravity. The latter is equivalent to a Brans-Dicke theory with a coupling parameter w=0, which is apparently excluded from the narrow interval, -3/2

Source: http://arxiv.org/abs/1001.0995v2

59
59

Sep 21, 2013
09/13

by
Francisco S. N. Lobo; Paulo Crawford

texts

######
eye 59

######
favorite 0

######
comment 0

Spherically symmetric thin-shell wormholes in the presence of a cosmological constant are constructed applying the cut-and-paste technique implemented by Visser. Using the Darmois-Israel formalism the surface stresses, which are concentrated at the wormhole throat, are determined. This construction allows one to apply a dynamical analysis to the throat, considering linearized radial perturbations around static solutions. For a large positive cosmological constant, i.e., for the Schwarzschild-de...

Source: http://arxiv.org/abs/gr-qc/0311002v1